1. Field of the Invention
The present invention relates to a magnetic material used in a high-frequency application and more particularly to a sintered body of hexagonal Z-type ferrite that can be used in electronic components such as a choke coil, a noise suppressing device, or the like, and/or in an electromagnetic wave absorber, in a high-frequency band between several MHz and several GHz.
2. Description of the Related Art
As the use of a portable phone, a wireless LAN (Local Area Network), a personal computer in high frequency bands increases in recent years, it becomes necessary that elements or components themselves mounted within electronic products can also operate in the high-frequency environments. However, despite these demands, conventional spinel ferrite cannot be used in the high-frequency bands due to the frequency limit called Snoek's limit. To solve this problem, hexagonal ferrite having an easy magnetization plane and a hexagonal crystal structure is under review so as to be used as a material that can overcome the frequency limit problem and can be used in high-frequency applications. It is known that the hexagonal Z-type ferrite containing Co in particular, among the hexagonal ferrite products, has comparatively high initial permeability and exhibits excellent high-frequency characteristics. When the hexagonal ferrite is to be used in electronic components such as coil components or the like, it is necessary that the hexagonal ferrite has high volume resistivity from the viewpoint of obtaining insulating property, in addition to the high initial permeability and excellent frequency characteristics.
To respond to the demands above, hexagonal Z-type ferrite that can be suitably used in inductance elements is disclosed in Japanese Patent Application Laid-open No. Hei 9-110432 which is manufactured by making Si (silicon) and Ca (calcium) be contained therein. Also, Japanese Patent Application Laid-open No. Hei 9-129433 describes that high-frequency characteristics can be improved by addition of Si and Ca.
Also, Japanese Patent Application Laid-open No. Hei 10-92624 describes that hexagonal ferrite having high resistivity can be manufactured by making Si and Pb or the like, in addition to oxides serving as main components, be contained therein to vitrify SiO2 and PbO. Similarly, Japanese Patent Application Laid-open No. 2001-39718 describes a high resistance effect and a method in which an oxide magnetic material being excellent in magnetic characteristics in high-frequency bands and having high specific resistance can be manufactured by performing a sintering operation after both Bi2O3 and Mn3O4 have been added simultaneously following a calcination process.
Japanese Patent Application Laid-open No. 2002-362968 describes a method of manufacturing ferrite with high resistance without using any additive in which ferrite having high specific resistance can be obtained by providing a low-cooling-rate region in cooling steps of sintering processes in which a cooling rate is 0° C./min or higher and less than 1° C./min. Moreover, Japanese Patent Application Laid-open No. 2003-2656 describes that resistance is made higher by blending 1% to 10% by mass of barium carbonate or strontium carbonate or both of them or 0.5% by mass of SiO2. Furthermore, Japanese Patent Application Laid-open No. Sho 50-32207 describes that a temperature characteristic of initial permeability can be improved by substituting Co3+ and Li+ for part of divalent ions of Z-type ferrite.
However, in the conventional hexagonal ferrite disclosed in the above references, it is impossible to manufacture hexagonal ferrite being excellent in high-frequency characteristics which can achieve both desired high permeability and desired high volume resistivity. That is, a ferrite sintered body of the conventional hexagonal Z-type ferrite cannot provide sufficiently satisfactory characteristics.